Field of the Invention
The present invention relates to a polythiophene derivative, and a secondary cell positive electrode active material using the polythiophene derivative and a secondary cell.
Description of the Related Art
Cells extract electric energy by converting chemical energy to electric energy utilizing oxidation-reduction reaction occurring at positive electrodes and negative electrodes, or store electric energy by undergoing a reverse process. Cells are used in various devices as power supplies.
Along with the recent rapid expansion of the market for notebook personal computers, smart phones, and so forth, there are increasing needs for drastic improvement of energy density and output density of the secondary cells used in these devices. Besides, in order to alleviate the difficulty in the power situation since the Great East Japan Earthquake, development of large-scale large-capacity secondary cells is highly hoped for. In order to meet this demand, secondary cells using alkali metal ion such as lithium ion as a charge carrier to utilize electrochemical reactions upon charge exchange by the charge carrier are being strenuously developed.
However, most electrode materials for positive electrodes (positive electrode active materials) of lithium ion secondary cells are poorer in discharge capacity (Ah/kg) than electrode materials for negative electrodes (negative electrode active materials). This is a large factor that hinders expansion of capacity of lithium ion secondary cells. Lithium ion secondary cells currently put on the market use metal oxides having high specific gravities as positive electrode active materials. Therefore, there is a problem that cell capacity per unit mass is not sufficient. Hence, attempts to develop large-capacity cells using more light-weight electrode materials are being explored.
For example, U.S. Pat. No. 4,833,048 and Japanese Patent No. 2715778 disclose cells using organic compounds having disulfide bond as positive electrode active materials. These cells function as secondary cells by letting the disulfide bond undergo 2-electron reduction during discharging to cleave the sulfide bond and let it react with metal ions in the electrolyte and change to two metal thiolates, and by letting the two thiolates undergo 2-electron oxidation and return to sulfide during charging. Because these secondary cells use organic compounds mainly containing elements having low specific gravities such as sulfur and carbon as electrode materials, these secondary cells are effective to a certain degree in terms of configuring large-capacity cells having a high energy density. However, there is a problem that efficiency of recombination of the dissociated disulfide bond is poor, so stability in the charging state or discharging state is insufficient.
As cells using organic compounds likewise as active materials, Japanese Examined Patent Publication No. 07-85420 discloses a cell using a polypyrrole complex and Japanese Unexamined Patent Application Publication No. 2002-304996 discloses a cell using a nitroxyl radical compound as a positive electrode active material. Japanese Unexamined Patent Application Publication No. 2002-304996 describes piperidyl group-containing high-molecular-weight polymers and copolymers as the nitroxyl radical compound.
Chemical Physics Letters, 359, (2002) 351-354 discloses a secondary cell using 2,2,6,6-tetramethyl piperidinoxyl-7-yl methacrylate (PTMA) as a positive electrode active material.
However, there is a problem that only a certain amount or less of charges can be injected into or discharged from conductive polymers such as polypyrrole because generated charges spread in the polymers to cause a strong Coulomb repulsion between charges. The nitroxy radical compound has an advantage of being able to obtain a large current because charge exchange at the electrode is rapid. However, the nitroxy radical compound is not suitable for expansion of capacity of secondary cells because the nitroxy radical compound undergoes oxidation-reduction reaction at a rate of one electrode per molecule.
Japanese Unexamined Patent Application Publication No. 2010-80343 and Nature Materials, 10, (2011) 947-951 disclose secondary cells using low-molecular-weight organic compounds having a multiple-stage redox ability as active materials. However, because of the low-molecular-weight compounds, which have high capacity densities though, these secondary cells have problems such as degradation of cell performances due to elution of the low-molecular-weight compounds into the electrolytes. Resolution of the problems is demanded.
When mounting an electrode formed of a current collector and an electrode layer in a secondary cell, the electrode layer needs to have a certain degree of flexibility in order that the electrode layer can be prevented from being detached from the current collector. Generally, a binder resin is mixed in producing an electrode layer in order to impart flexibility to the electrode layer. However, increase of the amount of the binder resin in the electrode layer is accompanied by decrease of the amount of the active material, to lower the capacity of the secondary cell. Therefore, it is demanded to provide an electrode having flexibility in the film even with a small amount of a binder resin.